Old English byrst "bristle," with metathesis of -r-, from Proto-Germanic *bursti- (source also of Middle Dutch borstel, German borste), from PIE *bhrsti- from root *bhars- "point, bristle" (source also of Sanskrit bhrstih "point, spike"). With -el, diminutive suffix.

c. 1200 (implied in past participle adjective bristled) "set or covered with bristles," from bristle (n.). Meaning "become angry or excited" is 1540s, from the way animals show fight. Related: Bristling.